PERSPECTIVES Also see the archival list of Science's Compass: Enhanced Perspectives PALEOCLIMATE: Enhanced: CO2 and Climate Change

Geologists have long known that on time scales of tens of millions of years, intervals of continental glaciation were interspersed with times of little or no ice [HN1]. The magnitude of warmth during these warm intervals is impressive. At times during the Cretaceous [about 65 to 145 million years ago (Ma)], duck-billed dinosaurs roamed the northern slope of Alaska. Deep and bottom waters of the ocean, now near freezing, could reach a balmy 15ºC. In the 1980s, a convergence of results from paleoclimate data and geochemical and climate models suggested that such long-term variations in climate were strongly influenced by natural variations in the carbon dioxide (CO2) content of the atmosphere [HN2] (1). Lately, some geochemical results have raised concerns about the validity of this conclusion. CO2 concentrations over the past 65 million years appear to have reached low levels well before the most recent phase (the past 3 million years) of Northern Hemisphere glaciation. This is especially true for times of elevated temperatures at about 50 to 60 Ma and 16 Ma, when CO2 was apparently low (2-4). A study spanning the Phanerozoic [HN3] (the past 54